If two nudists have sex on a nudist beach, is it illegal?

Yes, it would still be illegal because regardless of the legality of the nudity in that location, acts of public sex would still be illegal.  Nudist beaches have groups of the local regulars, often called the beach "ambassadors", who patrol the beach and keep an eye out for any sort of sexual activity, voyeurism, hidden cameras, etc. in order to remove those people from the beach (with police assistance as necessary) and keep the beach safe, comfortable, free, and open for all people who just want to spend a nice day at the beach enjoying the pure freedom of wearing nothing at all.  Just because nudity is legal there, that doesn't mean anything goes.

That depends on where the beach is and what the rules and local laws are.

Everybody should have a sex.

Generally yes, it is.

It's illegal......

Yes, it's illegal. Makes no difference who is doing it.

Public sex is illegal. However, if the beach is privately owned and no one but invited/paying guests can frequent it, and they know they may encounter people having sex, the sex is OK. It just has to be taking place in an area that non-guests can't inadvertently see.

I wouldn't arrest them. It's natural, right? And people go there to be natural . . .  it makes sense. I wouldn't do it in front of people, but that is just me.

I’m not sure, but I know for damn sure no oneness should do that in front of the public